FIG. 18 shows a known engine timing drive composed of a chain guide mechanism, a driving sprocket S1 mounted on a crankshaft, a pair of driven sprockets S2 and S3 mounted on valve-operating camshafts, an endless timing chain CH engaged with the driving and driven sprockets, a pivoting chain guide 520 for guiding and keeping adequate tension in the timing chain, a tensioner T for exerting a force pressing the guide 520 against the span of the chain traveling from the driving sprocket S1 to driven sprocket S2, and a fixed chain guide 530 in sliding engagement with the span of the chain traveling from driven sprocket S3 to driving sprocket S1 for guiding the travel of the chain. The timing drive is typically disposed within a timing drive compartment (not shown) formed by an engine block and a timing chain cover.
The pivoted chain guide 520 is pivotably mounted on a mounting shaft B fixed to the engine block, and the fixed chain guide 530 is attached to the engine by mounting shafts B3 and B4, which are also fixed to the engine block. In assembly of the timing drive, and in disassembly of the timing drive for maintenance or replacement, the driving sprocket S1, the driven sprockets S2 and S3, the pivoting chain guide 520, the fixed chain guide 530, and the timing chain C, need to be mounted or dismounted individually. Consequently the process of assembly and disassembly is difficult and time-consuming.
To alleviate the aforementioned difficulties in assembly and disassembly of a timing drive, another known timing system has been provided in which the pivoted and fixed chain guides are connected by a guide bridge. As shown in FIG. 19, in the chain guide mechanism of this known timing system, the fixed chain guide 630 is formed as a unit with a guide bridge 610. A mounting hole 612, located adjacent an end of the guide bridge 610 remote from the fixed guide 630, receives a mounting boss 621 on a pivoted guide 620, which in turn receives a mounting shaft B on the engine block. The mounting shaft B, the boss 621, and the mounting hole 612, form a fulcrum about which the guide 620 pivots.
The guide bridge 610 is formed so that it defines the relative positions of the driving sprocket S1, the driven sprockets S2 and S3 and the pivoting chain guide 620. The positioning of these components during assembly or maintenance is thereby simplified. A provisional assembly can be made before the components are mounted on the engine, as shown and explained in Japanese laid-open Patent Application No. Hei. 11-63128.
While the known chain guide mechanism in FIG. 19 integrates the pivoting chain guide 620 and the fixed chain guide 630, it only defines the relative positional relationships of the driving sprocket S1, the driven sprockets S2 and S3, and the timing chain C. It does not integrally hold the sprockets and chain. Consequently, the improvements afforded by the guide bridge 610 are relatively minor.
Another problem with the guide bridge 610 is that, because the part thereof that connects the fixed and pivoted guides extends across the timing chain only on the side of the timing chain CH remote from the engine block, the mounting hole 612 can become disconnected from the boss 621 of the pivoting guide 620 if the guide bridge 610 is deformed in the direction of its thickness, as shown in FIG. 20.